The Basics

What Contributes To Silica Deposition In Water Systems?

By Nikhilesh Mukherjee

Deposition Scaling Silica Particles

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Abstract

Silica deposition is a serious problem in the water industry. In high-purity water, it is damaging to device manufacturing in microelectronics plants, so facilities follow standards to keep silica levels in the sub-parts per billion (ppb) levels. In steam power generation, silica can form deposits on heat-exchange surfaces where it reduces thermal efficiency.In high-temperature boilers at 600 to 900°K and 1-100 atmosphere (atm) steam pressure, Si(OH)4 is the most likely silicon-containing gaseous species. This form of silica can volatilize volatilize and carryover with steam where it can form deposits on turbine blades and lower aerodynamic efficiency. In cooling water, the presence of silica limits the cycles of concentration and can significantly increase the cost of water pretreatment. This article reviews factors that contribute to silica scaling in industrial and high-purity water systems.

Let us start from the basics.

What is SiO2?

Silicon dioxide (SiO2) is not really a molecule, at least when it is found in nature. It is the repeating unit of a giant 3-D covalent structure called silica. SiO2 is an empirical way of expressing the presence silicon (Si) in SiO2 in which each every Si atom is surrounded by 4 oxygen (O) atoms by covalent bonds and every O atom is surrounded by two Si atoms by covalent bonds. Thus 4 O atoms are shared by 2 Si atoms in large crystal structure of silica with the ratio of silicon to oxygens  being 1:2. This gives the  ratio of Si : O = 1:2 and a molecular formula SiO2.

 

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Revised toray june 18.upm digital

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